The Advanced Television Systems Committee (ATSC) published a Digital Television Standard in 1995 as Document A/53, hereinafter referred to simply as “A/53” for sake of brevity. Annex D of A/53 titled “RF/Transmission Systems Characteristics” is particularly pertinent to this specification. In the beginning years of the twenty-first century efforts were made to provide for more robust transmission of data over broadcast DTV channels without unduly disrupting the operation of so-called “legacy” DTV receivers already in the field. These efforts culminated in a candidate ATSC standard directed to broadcasting digital television and digital data to mobile receivers being adopted on 15 Oct. 2009. This standard, referred to as “A/153”, is also pertinent to this specification.
A/153 prescribes forward-error-correction coding of data transmitted to mobile receivers, which FEC coding comprises transversal Reed-Solomon (TRS) coding combined with lateral cyclic-redundancy-check (CRC) codes to locate byte errors for the TRS coding. This FEC coding helps overcome temporary fading in which received signal strength momentarily falls below that needed for successful reception. The strongest TRS codes prescribed by A/153 can overcome such drop-outs in received signal strength that are as long as four tenths of a second.
Another known technique for overcoming temporary fading is iterative diversity. Iterative diversity can also overcome certain types of intermittent radio-frequency interference. Communications systems provide for iterative diversity of received signals by transmitting a composite signal composed of two component content-representative signals, one of which is delayed with respect to the other. The composite signal is broadcast to one or more receivers through a communications channel. At a receiver, delayed response to the initially transmitted component content-representative signal supplied from a buffer memory is contemporaneous in time with the finally transmitted component content-representative signal. Under normal conditions, the receiver detects and reproduces the content of the finally transmitted signal as soon as it is received. However, if a drop-out in received signal strength occurs, then the receiver detects and reproduces the content of the initially transmitted signal as read from buffer memory. If the delay period and the associated delay buffer are large enough, then fairly long drop-outs in received signal strength can be overcome. This capability not only requires a severalfold increase in the amount of memory required in a receiver; it halves the effective code rate of the transmission. However, drop-outs as a long as a few seconds are feasible using memory of the sizes used in flash drives.
In a June 2007 ATSC subgroup meeting Thomson, Inc. described forms of iterative diversity its engineers called “staggercasting” for use in robust portions of 8-VSB transmissions. Thomson, Inc. advocated iterative diversity in which the earlier and final robust transmissions of the same data are combined in the “transport” layer of the receiver. The “transport” layer of the receiver is subsequent to the “physical layer” of the receiver, which “physical layer” comprises the initial stages of the receiver that recover transport-stream packets from the robust portions of 8-VSB transmissions. Transport stream (TS) packets from the earlier one of the iterated transmissions replace missing TS packets in the later one of the iterated transmissions in staggercasting. Thomson, Inc. and Micronas GmbH jointly proposed a concatenation of outer block coding with inner ⅔ trellis coding per 8-VSB for each component transmission. A representative of the two companies pointed out that earlier and final transmissions of the same coded data could be combined along the lines used in digital audio broadcasting (DAB) to implement iterative diversity reception. The representative suggested transmitting the same coded data by two or more 8-VSB transmitters to facilitate frequency-diversity reception by M/H receivers that combined transmissions of the same coded data in their physical layers. Later on, Thomson, Inc. and Micronas GmbH representatives returned to advocating the combination of decoded earlier and final transmissions of the same data in the transport layer of the receiver, rather than attempting to combine those earlier and final transmissions in the physical layer during decoding procedures.
The day of that ATSC subgroup meeting Samsung Electronics Co., Ltd. disclosed specific designs for receivers in which the earlier and final robust transmissions of the same data were combined in the “physical layer” of the receiver so as to achieve decoding gain during iterative-diversity reception. These early receiver designs are described in U.S. patent application Ser. No. 12/228,959 filed 18 Aug. 2008 for A. L. R. Limberg, titled “Staggercasting of DTV signals that employ serially concatenated convolutional coding” and published 26 Feb. 2009 as A1 U.S. Pub. No. 2009-0052544. Outer convolutional coding combined with inner ⅔ trellis coding per 8-VSB to create serial concatenated convolutional coding (SCCC) for each component transmission, the SCCC being of a sort earlier described by Dr. Jung-Pil Yu of Samsung. These early receiver designs decoded the inner convolutional coding as received at separate times, but deferred decoding of the outer convolutional coding of earlier CCC transmissions so it could be performed contemporaneously with the decoding of the outer convolutional coding of final CCC transmissions. The contemporaneous decoding procedures for the outer convolutional coding was accompanied by an exchange of interim decoding results between the decoding procedures to increase decoding gains for decoding the outer convolutional coding. Later receiver designs delayed digitized earlier transmissions before turbo decoding those transmissions contemporaneously with digitized later transmissions. Interim decoding results were exchanged between the decoding procedures to increase turbo decoding gains. These later receiver designs are described in U.S. patent application Ser. No. 12/580,534 filed 16 Oct. 2009 for A. L. R. Limberg, titled “Digital television systems employing concatenated convolutional coded data” and published 22 Apr. 2010 as A1 U.S. Pub. No. 2010-0100793.
In an on-channel distributed transmitters system (DTS) the relative magnitudes of individual signals can differ by only a fraction of a decibel over a wide area, and their differential delays can range to far more than ten microseconds. Usually, the echoes of each signal are smaller than the signal itself, and the delay of each significantly large echo and the signal itself is less than ten microseconds. The DTS can comprise one centrally located high-power transmitter with a high-gain antenna that is as high as possible and one or more on-channel, lower-power transmitters with lower-height towers and lower-gain antennas. The single-frequency network (SFN) is a special case of DTS and is composed of a network of transmitters designed so that all transmitters have similar-gain antennas and similar low ERP. SFNs can be used successfully with multi-carrier signals using COFDM (Coded Orthogonal Frequency Division Multiplex). Some members of ATSC have advocated SFNs using 8-VSB signals, and ATSC document A/110B “Synchronization Standard for Distributed Transmission; Revision B” is directed to synchronous transmissions from a plurality of 8-VSB transmitters. The FCC has advocated SFNs for 8-VSB transmitters as a way in which to conserve broadcasting frequency spectrum.
These efforts are ill conceived, however, because they fail to take into consideration the fact that there is a fundamental problem with distributed transmissions using a single-frequency carrier in common. Many receivers in the zones of overlapping coverage areas of the 8-VSB transmitters will experience nulls in the frequency spectrum of received signals that prevent satisfactory reception. Attempts to equalize the frequency spectrum will raise the noise in the null portions of the spectrum, interfering with automatic gain control of the radio-frequency (RF) and intermediate-frequency (IF) amplifiers of a receiver and being apt to reduce signal-to-noise ratio (SNR) too much for reception to be satisfactory. Indeed, SNR may be reduced so much that reception is not obtainable at all. Nulls in the portion of the frequency spectrum containing pilot carrier can reduce carrier-to-noise ratio (CNR) so low that the 8-VSB signal cannot be synchronously detected successfully.
Some proponents of DTS broadcasting have speculated that these problems can be overcome by using directional antennas at the receiver, but this would in any case be impractical for M/H receivers. M/H receivers are apt to change their reception sites during operation, presenting changing demands for reception antenna directionality. Hand-held receivers that use complex antenna arrays instead of a simple antenna are likely to be too cumbersome to carry around.
The proponents of on-channel distributed transmitters systems envision increased CNR and SNR being obtained by combining 8-VSB signals from a plurality of transmitters in the analog regime before the M/H receiver amplifies those signals. The combining of the 8-VSB signals in the analog regime is done in the reception antenna or directly thereafter. Unfortunately, this paradigm for combining 8-VSB signals from a plurality of transmitters does not result in increased CNR and SNR at all locations in the regions where the coverage areas of the transmitters overlap.
The inventor points out that there are different paradigms available for combining 8-VSB signals from a plurality of transmitters, providing that the transmitters broadcast the same information on different carrier frequencies to facilitate frequency-diversity reception. Broadcasting the same information on different carrier frequencies fits with the current methods for allocating television channels, in which transmitters in adjacent markets use different carrier frequencies and broadcast with sufficient power as to have overlapping coverage areas. Broadcasting the same information on different carrier frequencies fits with the current methods of filling gaps in coverage areas with low-power repeater transmitters. Broadcasting the same information on different carrier frequencies avoids having to expend the large amount of capital required to implement DTS broadcasting.
If two transmitters broadcast the same information over different RF channels, using different carrier frequencies to facilitate frequency-diversity reception, separate tuners can be used to recover respective baseband 8-VSB signals from the two radio-frequency 8-VSB signals. Channel-equalization filtering for each of the RF channels is done separately, with simpler echo patterns and consequently less reduction in SNR than would be expected with the shared channel-equalization filtering for DTS. The 8-level symbols in the two baseband 8-VSB signals are similar, but the noise spectra are unrelated. When a receiver is situated in the zone of overlapping coverage so the demodulated noise spectra have similar strengths, simply combining the two baseband 8-VSB signals in the analog regime before data slicing can improve SNR for AWGN by as much as 3 dB. (Alternatively, the signals can be over-sampled during digitization and combined before data-slicing.) This gain in SNR can be stably achieved and is equal to the most gain to be expected from RF 8-VSB signals from two on-channel DTS transmitters being combined in the analog regime at the reception antenna. Respective phase shifts in the carrier waves of the RF 8-VSB signals that are caused by different receiver locations and changing multipath conditions can be individually suppressed by the 8-VSB signals from separate RF channels being separately synchrodyned to baseband. This separate phase-tracking of the carrier frequency components of the 8-VSB signals to be combined is not available to a pair of 8-VSB signals with similar non-zero carrier frequencies, so combining DTS signals at the reception antenna is problematic. Small changes in reception antenna positioning or multipath conditions are likely to defeat satisfactory reception if DTS signals from two or more on-channel DTS transmitters are combined at the reception antenna.
Simply combining two 8-VSB signals before decoding them is advantageous only so long as neither of them is obliterated by burst noise of substantial duration or impulse noise of only a few symbols duration. More sophisticated approaches are possible in which the two 8-VSB signals are contemporaneously decoded using respective soft-input/soft-output decoders, which SISO decoders exchange information concerning data bits and the confidence levels of those bits during the course of reiterative decoding. Coding gains can be achieved that improve SNR for AWGN without being as affected by one of the 8-VSB signals being obliterated.
A pair of 8-VSB signals received from respective transmitters located at different sites are likely to be differentially delayed when received by an individual receiver located in the zone where the coverage areas of the transmitters overlap. When the pair of 8-VSB signals are received from an on-channel DTS and combined at the antenna or shortly thereafter, the differential delay reduces CNR and creates psuedo-echoes that the adaptive channel-equalization filtering should correct. Correction of these psuedo-echoes causes a substantial reduction in SNR if neither of the pair of 8-VSB signals is substantially stronger than the other. When two 8-VSB signals modulated with the same data are received via separate RF channels, adaptive channel-equalization filtering of each 8-VSB signal is separately performed before the 8-VSB signals are processed as a pair. Neither 8-VSB signal causes pseudo-echoes in the other that should be corrected by the adaptive channel-equalization filtering of that other 8-VSB signal. So, SNR reduction in the adaptive channel-equalization filtering of each 8-VSB signal will usually be lower than the SNR reduction in the adaptive channel-equalization filtering of the combined 8-VSB signals broadcast by an on-channel DTS.
Two tuners are required for frequency-diversity reception of the same main-service programming as contemporaneously transmitted by two 8-VSB transmitters broadcasting over respective RF channels separate from each other in frequency. Each tuner is typically composed of an R-F amplifier, an RF-to-IF converter, an IF amplifier, an analog-to-digital converter, synchrodyning apparatus for demodulating the 8-VSB signal, and an adaptive channel-equalization filter. Two tuners can also be used for frequency-diversity reception of the same M/H-service programming as contemporaneously transmitted by two 8-VSB transmitters broadcasting over respective RF channels separate from each other in frequency. Using two tuners in “mobile” receivers built into a vehicle is a not much of a problem beyond added cost for the extra hardware. Using two tuners in “handheld” receivers not only adds to the cost of the hardware, but causes unwanted additional drain of power from its battery supply, shortening the time of usage between recharging or switch-out of the battery supply.
The inventor observed that “handheld” receivers are designed for primarily receiving M/H-service programming rather than main-service programming. The inventor further observed that the same M/H-service programming can be broadcast by separate 8-VSB transmitters over different RF channels at somewhat different times, the earlier CCC transmission being delayed and then turbo decoded contemporaneously with turbo decoding of the later CCC transmission. That is, frequency-diversity reception invariably is linked with some degree of iterative-diversity reception. Respective turbo decoders for the earlier and the later CCC transmissions exchange information concerning data bits and the confidence levels of those bits during the course of re-iterative decoding. Coding gains can be achieved that improve SNR for AWGN. Iterative-diversity and frequency-diversity reception can be combined in such a way that a single frequency-agile front-end tuner rapidly switching its tuning between the different carrier frequencies of the two 8-VSB transmitters can receive both the earlier and the later CCC transmissions.